The present invention refers to an improved infrared heating oven for conditioning individual items moving substantially in a continuous manner and in an aligned arrangement inside said oven. In particular, the present invention applies preferentially to moulding machines capable of moulding at the same time a plurality of plastic items, which are generally known as xe2x80x9cpreformsxe2x80x9d in the art and are intended for subsequent blow-moulding into appropriate Final containers, especially plastic bottles.
The machines used to mould the final products, ie. to blow mould said preforms into said final containers, are usually defined as xe2x80x9csingle-stage machinesxe2x80x9d, since they are capable of carrying out the whole process needed to convert the raw plastic material from the pelletized form in which it is fed to the machine, into the finished product, ie. said final containers.
However, to the purposes of the present invention, also machines used to only convert previously moulded preforms into final containers can be considered as single-stage machines, since the blow moulding process must anyway be preceded by a phase of temperature conditioning of the preforms, ie. the phase that is exactly the subject covered by the present invention.
The difference existing between single-stage and two-stage machines in this particular field is well-known to all those skilled in the art, so that it shall not be explained here any further.
For reasons of greater convenience, all those who may have an interest in this connection should refer to the patent application PCT/EP98/08380 and the patent EP 0 768 165 of this same Applicant.
It is a generally known fact in the art that, before going on through the actual blow moulding process, preforms are heated up to an optimum predefined temperature for blow moulding, and this is usually carried out in appropriate conditioning ovens that make use of infrared lamps as heating means.
The neck portion of said preforms must anyway remain cold, ie. at a lower temperature than the glass transition temperature, so that the subsequent blow moulding operations will not cause it to distort.
Also well-known is the fact that the heating of the body of the preforms is obtained by the combined effect of:
the infrared radiation penetrating the thickness of the body of the preform (convection),
the contact with the heated ambient air of the interior of the oven (conduction), and
heat diffusion by inner conduction in the wall of the preform (thermal stabilization/inversion).
It is a commonly felt need for the interior of the oven and the outer surface of the preforms arranged therein to be ventilated with a flow of air at a pre-established, anyway relatively low temperature, so that the material lying on the outer surface of the preforms is prevented from being brought up to an excessively high temperature that would cause it to crystallize, while waiting for the material lying inside the preforms to reach the required blow-moulding temperature.
Such a ventilation enables the ambient temperature inside the oven to be kept at a desirable level, while eliminating any excessive heat due to infrared rays that fail to be captured, ie. retained by the preforms themselves, as well as the xe2x80x9cskinxe2x80x9d temperature of the preforms to be mitigated.
Such a desirable level of the internal temperature of the oven is the one that actually enables the energy efficiency of the oven to be optimized. It is in fact a largely known point that it is a function of the thickness of the preforms.
The ventilation of the oven must be carried out at an adequate flow rate in view of being able to satisfactorily perform such a function, and it is desirable that the ventilation itself is well distributed all over the interior of the oven in such a manner as to ensure that the whole surface of the wall of the preform, which must be heated up, is actually treated in an even, homogeneous manner.
It is also necessary for the ventilating air, which is heated up as it flows through the oven, to avoid to touch, ie. flow over the neck of the preforms, which on the contrary should remain as cold as possible.
This problem of being able to ensure a well-balanced heating of the preforms, without anyway involving the neck portion thereof, has been the subject of a number of investigation and research works, as well as a corresponding number of proposed solutions, as documented by the various existing prior-art patent publications covering this matter, among which the following ones are cited here:
U.S. Pat. No. 5,256,341 in the name of SIDEL; according to this document, the heating of the preforms is obtained both directly and indirectly by making use of the reflective properties of a core that is placed inside each single preform during the temperature conditioning operation. This solution certainly provides some advantages as far energy efficiency is concerned, but involves considerable construction and operating complications connected with the need for said cores to be handled and driven in a synchronized manner; furthermore, the ability to comply with the above cited requirement calling for the neck portion of the preforms to be as far as possible guarded against heating is not adequately ensured.
U.S. Pat. No. 4,923,395 in the name of HUSKY INJECTION MOLDING SYSTEMS LTD. discloses a solution according to which a kind of heating action is provided which is aimed at, ie. focussed on the preforms themselves by means of a variable positioning of the battery of heating elements, in such a manner as to generate a flow of infrared radiation that exactly corresponds to the peculiar requirements arising in connection with each particular type of preforms being handled (thickness, material, outer dimensions and so on). However, this solution proves quite difficult to be implemented from a practical point of view, owing to the really considerable contraindications of a construction-related and operating nature arising also in view of being able to avoid irradiating the necks of the preforms, and the high costs that are generally connected thereto.
U.S. Pat. No. 5,066,222 to BEKUM MASCHINENFABRIK GmbH discloses in turn the solution consisting in spacing the preforms from each other as they move through the oven along the heating lamps, as well as protecting the neck portion of the same preforms with the use of thermally insulating sheaths capable of being removed after the temperature conditioning operation. However, this solution appears to anyway share some peculiarities and features with the above described one, so that it maintains and confirms definite construction-related and operating contraindications, as well as a related significant extent of associated costs.
U.S. Pat. No. 4,606,723 to SIDEL: this solution has a number of technical drawbacks that may be summarized as follows:
a) in the first place, the flow of cooling air only marginally passes over, ie. touches the lamps to cool them down, so that the need actually arises for the rate of the same flow to be correspondingly increased, thereby bringing however about the problem connected with the preforms that are not exposed to the heating radiations undergoing actually to an excessive cooling down;
b) in the second place, the bottom of the preform, ie. the closed portion opposing the neck thereof, fails to be irradiated by the upper lamp to any adequate extent, so that it undergoes an uncertain, generally unsatisfactory heating effect; if the number of these lamps is going to be increased in view of doing away with such a problem, this would on the other hand give rise to other problems, since the lamps themselves would in this case be exposed to a greater extent to the flow of cooling air and, therefore, would show different radiating characteristics;
c) owing to the asymmetry of the oven construction and the circulation of the air flow, the preforms, owing to them being invested by the air in a longitudinal downward direction, ie. from top to bottom, and not in a crosswise direction, would be conditioned in an inhomogeneous and, therefore, incorrect manner;
d) finally, in the described solution there is practically no possibility whatsoever for the sockets of the heating elements to be cooled down by the same flow of air used to cool down the body of the preforms; however, since these sockets anyway require to be cooled down in a rather intensive manner, the need therefore arises for a special cooling system to be provided and this of course leads to a greater complexity in the construction of the plant and correspondingly higher costs.
U.S. Pat. No. 5,322,651 to SIDEL SA teaches to heat up in a controlled manner a plurality of preforms cooled down by means of a flow of air whose temperature is controlled owing to it being the result of the air exhausted from the oven being mixed with fresh outside air, in such a manner as to obtain cooling air at always the same temperature. This solution, however, refers, as far as other technical and functional aspects of the matter are concerned, to substantially the same architecture and operating criteria illustrated in the afore cited patent publication, so that it is unavoidably associated to the same kind of drawbacks illustrated in connection with the afore mentioned solution.
Furthermore, it seems at this point quite appropriate for a rather simple, although important consideration to be made: in order to obtain a rather constant flow of cooling air, the need arises for the related conduit to have a cross-section area that is as large as possible.
On the contrary, in order to achieve a satisfactory reflection of the infrared-ray radiation by the walls of the same conduit towards the preform contained therein, the need arises for the conduit itself to have, at least in the zone lying close to the preforms, a very narrow cross-section area.
Therefore, between these clashing requirements there is an obvious contrast, which does not seem to have been done away with to any satisfactory extent by any of the prior-art solutions.
Based on the above considerations, it is therefore a main purpose of the present invention to provide a preform conditioning oven that is free from the afore described drawbacks, and is further reliable, capable of being implemented in a simple manner using readily available techniques and materials, and is capable of solving by a satisfactory compromise the antagonistic requirements of a constant air flow and an adequate reflection efficiency of the inner walls of the conduit.
Such an main of the present invention, along with further features thereof, is reached in an oven that is made and operates as recited in the appended claims.